Gyatso didn't just love Aang because he was a good monk.
He watched his best friend get taken by the Avatar burden their whole life. Roku never really got to just be a person, every decision, every relationship came second to being the Avatar.
When Aang came, Gyatso wasn't going to let that happen again.
That's why he fought so hard to keep Aang away from the other monks and let him have some fun. Not guilt, it was a second chance to protect someone he already lost once.
They shared same laugh, same mischief and same soul but in different bodies